Exposure of proteins to oxygen and, especially, to reduced oxygen compounds, such as hydrogen peroxide and hydroxyl radical, permits the oxidation of sensitive sites on the protein molecules. Of the sensitive sites, the sulfur of methionine is particularly subject to attack. An enzyme, methionine sulfoxide reductase, can reverse this oxidation and it has been shown to be a player in the continuing repair of oxidatively damaged proteins. Assays for this enzyme have shown that there is a steady, if undramatic, loss in activity in livers of rats of increasing age, falling at 24 months to values 80 percent of those at 2 months. These findings suggest that one of the effects of aging is decreased ability to repair damaged enzymes. Surveys of extracts of different organs show that the enzyme activity is widely distributed in the rat with the greatest activity found in the kidney. Such distribution supports the hypothesis that the enzyme is important in the maintenance of the cell. The high level of activity in the kidney may represent the utility of the enzyme in reclaiming oxidized methionine and thereby sparing the nutritional demands for replacement sources. Methionine sulfoxide reductase has been purified from Escherichia coli. Whereas there is a low level of activity in cultures of E. coli during growth, the levels rise dramatically in stationary phase.